It is a particular problem in the handling of webs such as textiles that certain treatments can cause lateral distortion, especially shrinkage. In order to counter this, web-handling machinery is conventionally provided with `temples` which maintain the lateral extent of the web against shrinkage, warping, etc. by preventing the edges of the web from moving inwards during the weaving process, finishing process and general handling.
Such restraint can frequently lead, however, to splitting of the web, especially when the weft threads (in a woven web) are relatively weak. A particular area where this problem has been encountered is in the production of a tire cord sheet (ply stock) for use in the reinforcement of tires. A typical cord sheet or ply stock comprises a long (typically up to 3000 m) strip of fabric made up of a warp of closely spaced cords, e.g. twisted synthetic filament yarns or wire, with relatively weak and widely spaced cotton weft strands serving to maintain the warp cords in a parallel even array. Each end of a cord sheet of this type comprises a `tabby`--a short length which is reinforced with densly woven weft strands, conventionally of cotton yarn. The term `tabby` is used herein to denote the end portion of a length of tire cord sheet, otherwise known as a splicing header, splicing head-end or bordure which is used to handle the cord sheet and, in particular to join sheets together into a long web for further processing. The further processing involves applying longitudinal tension and dipping in an aqueous latex bath and rapid hot-air drying and heat setting in an ascending column drier, thus subjecting the cotton to shrinkage caused by heat and wet. During the dipping process the cord sheet passes over banana rollers and the like which both extend and narrow the width of the sheet, causing non-resilient weft threads to break, leading to splitting or distortion of the sheet. The latexed sheet is then subsequently coated with rubber between calender rolls and split to provide the reinforcement ply material used in tire manufacture.
The main effect of the tabby shrinkage is that the cord sheet becomes distorted over a considerable length near the tabby, with cords moving out of their generally two-dimensional array and becoming superimposed and tangled. The net result of this is that individual ends (warp strands) have different tensions; the spacing between them is irregular and the end product (i.e. the casing material) contains weaker or stronger points, which can seriously effect the safety of the tyre. This means that frequently a portion of the sheet has to be discarded after passing through the subsequent coating step.
Various ways of stabilising the tabby have been tried, including the replacement of cotton by other materials but most conventional yarn materials are subject to distortion or shrinkage during the dipping, drying and heat setting.
Thus, cotton shrinks and is weakened under aqueous conditions. Synthetics such as polyester and polyamide shrink and set under the action of heat. Alternatives such as glass or wire are too smooth for weaving in a tabby weave and would not bind well enough to the warp strands. There is thus a need for a material which can be incorporated in tabbies and which has the resistance to shrinkage under wet and/or hot conditions; does not become permanently heat set; which binds and hence can be successfully woven; and, importantly, in tire cord construction, which can adapt to the changing width of the cord sheet so that the tabby remains the same width as the sheet. The last point is a problem because changes in the width of the sheet can occur by reductions in the widths of the gaps between successive warp strands, but similar changes cannot occur to the same extent in the densely woven tabby.